Motor vehicles typically have a driver's seat that may be moved forward and rearward so that a driver is able to position the seat at a desired distance from the controls. The seat may, for example, be mounted on a track or rail that runs along the longitudinal (forward/rear) axis of the vehicle. As is well known in the vehicle seating art, a power seat mechanism may use an electric motor to move the seat along the track. The driver is thus able to adjust the seat position by simply moving a manual switch. Other seats in a vehicle may also be adjustable in this manner, most commonly the passenger seat next to the driver's seat in the front seating row of the passenger cabin.
Occupant safety systems sometimes use the forward/rearward position of the driver and/or front seat passenger as a factor in making decisions regarding activation of occupant restraints. For example, a frontal collision air bag may undergo a condition change in response to seat position. For example, the airbag may be deactivated or inflated less rapidly and/or less forcefully if the seat is forward of a reference position. In such systems, the seat position is typically detected by a sensor that indicates only two states: forward of the reference position or rearward of the reference position. These two-state or binary sensors change state only once during travel of the seat along the length of the rail on which the seat moves.
In at least one known system using a two-state sensor, a Hall-effect sensor is located on or adjacent to the seat track and detects the presence of a metal component on the movable seat frame when the seat passes by the sensor.
Some vehicle seating systems utilize an absolute seat position tracking system. In this context, “absolute” refers to the ability to identify the position of the seat at any spot along the range of forward/rear movement, rather than just forward or rearward of a reference location.
Tracking of the absolute seat position may be used to enable a seat position memory function, and/or entry/exit function wherein the seat is automatically moved rearward when the ignition key is removed from the switch and/or the driver door is open.
Some occupant safety systems also may use the absolute seat position to optimize decisions regarding the activation of passenger restraints (and/or other safety systems) with the goal of providing the most safety benefit during a collision or other incident.
Known absolute seat positions sensors utilize a continuously variable resistor and/or magnets to determine seat position along the entire length of the seat travel. However, such sensors are relatively large and heavy and so may be difficult to integrate into the seat frame/drive mechanism because of the limited package space.
Other sensors have been proposed which measure rotational movements of the shaft of a motor driving seat, but asynchronous movement between the seat and motor can occur due to elastic coupling of the drive mechanism. Such elastic coupling may be due to slippage or “play” between various components of the seat drivetrain. Such asynchronous movement makes it necessary to recalibrate the measurement after a period of operation in order to maintain accuracy.
Knowledge of the absolute seat position, as opposed to the binary or two-state position, may be used to advantage in occupant safety systems in many ways. For example, the absolute seat position may be used (by itself or in combination with others factors) to infer the size of the occupant of the seat (height and/or weight). This occupant size inference is based upon the assumption that a person of smaller stature is more likely to position the seat farther forward (in order to comfortably reach to the controls) and a larger statured person is more likely to position the seat farther to the rear. Both the size of the occupant and position of the occupant relative to the interior of the vehicle may be considered by a Restraint Control Module (RCM) in making decisions as to the deployment or activation of the passenger restraints (and/or other safety systems) in a manner designed to provide the most safety benefit during a collision or other incident.